Messenger RNA localization is a well-documented phenomenon and provides a mechanism by which to generate cell assymetry (St. Johnston, Cell 81:161-170 (1995); Glotzer et al., Cell Dev. Biol. 7:357-365 (1996); Steward et al., in mRNA Metabolism and Posttranscriptional Gene Regulation, Wiley-Liss, New York, 127-146). Messenger RNA localization has been studied by fluorescence in situ hybridization (FISH) (Long et al., RNA 1:1071-1078 (1995). In situ hybridization, and other methods that require fixation of cells, offer good spatial resolution, but are severely limited in temporal resolution. Thus, while these techniques are well-suited for determining where RNA goes in living cells, they are unsuited for determining how quickly, or by what route, the RNA travels to its destination.
A further limitation of FISH methods, is that fixation kills cells. Therefore, those methods are incompatible with cell selection, where cells must be kept alive to initiate a new cell line.